Well, I don't know if you can say anything about all conifers, but certainly some use C3 metabolism. It would make no sense, for example, for a pine tree in Northern Russia to use the C4 pathway as the C4 pathway evolved as a pathway to reduce photorespiration when it's hot. But, as some conifers live in hot climates, it may be totally possible for some to have C4 metabolism. I know for sure that there is one(and only one) conifer that uses the CAM pathway: Welwishia.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter